Nature has created a variety of biomass materials with various characteristics. However, all biomass materials are generally comprised of cellulose, hemicellulose and lignin. In particular, the category of biomass generally known as “wood” is comprised of approximately ⅓ by weight cellulose, approximately ⅓ by weight hemicellulose and approximately ⅓ by weight lignin. Because biomass is renewable and sequesters its carbon dioxide emissions, there is great interest in using biomass as a fuel. Biomass fuels are very low in sulfur, but have high moisture content and relatively low heating value per unit of weight, when compared to coal, for example. In order to create an efficient biomass-to-energy conversion, therefore, it is known to dry or to torrefy biomass materials for subsequent densification. Drying of biomass material will generally reduce the moisture content from 30-50% by weight in the green state to about 5-12% by weight, and it will increase the heating value per unit of weight because of fee loss of moisture weight, but it will not impart, significant chemical changes to the biomass material. Further heating under controlled conditions results in torrefaclion, which may be generally described as a pyrolysis or thermal degradation process carried out on wood or other biomass materials at temperatures typically less than about 600° F. in a low-oxygen atmosphere. During torrefaction, the hemicellulose in the biomass is broken down into volatile organic compounds (“VOCs”) such as methanol, formaldehyde and acetaldehyde, and at least a portion of these volatile organic compounds are released by evaporation. The removal of hemicellulose increases the energy density or heating value per unit of weight of the biomass; however, full torrefaction without recovery of the evaporated volatile organic compounds results in the loss of the energy stored therein.
A pelletized form of torrefied wood or biomass is highly desirable in that it would allow easy and relatively clean handling and more efficient shipment, especially ocean freight shipment, of a biomass-based fuel. Many attempts have been made to form fuel pellets of torrefied wood and biomass. However, the inventors are not aware of any devices or methods for torrefying or otherwise preparing green biomass to be pelletized that do not require that the processed biomass material be pelletized with an additive, such as lignin, to form a stable pellet.
It would be desirable if a process and apparatus could be developed for making moisture-resistant torrefied fuel pellets from biomass materials without requiring the addition of a binder additive. It would be desirable if such a process and apparatus could be developed for producing a fuel pellet from torrefied biomass that has heating values similar to those of coal, as well as high grindability, structural stability, and moisture resistance. It would also be desirable if the same apparatus could be employed to process biomass materials for use in making white biomass pellets.